What makes gravity wave?

Occasionally something comes along which is as exciting as travelling, and makes me want to write off-topic. Newspapers have been full of the story of the discovery of gravitational waves. One corner of Mumbai is full of boffins who know all about this. I tried to find out from experts why this is interesting. This post is a broad-brush picture of what I now understand.

When I throw an apple upwards, it does not continue to move in a straight line. Its velocity changes: the upward motion reverses and becomes a downward motion. Newton realized that this change in velocity (which he named acceleration) means that some force acts on the apple. This is the force of gravity.

The planets go around the sun. This means that each of them is constantly changing its direction of motion (think of going round a tree: at some time you may be moving east, halfway around you will be moving west; the direction of your motion has changed). This change in velocity must also be due to a force. Newton ascribed this change to the sun’s gravity. Then he computed how the gravitational force depends on the distance from the sun, and came to the famous conclusion that it changes as the inverse square of the distance.

Gravity is a force field: everywhere in space the sun exerts a force. This has different strengths at different places, but always pulls in towards the sun. We are used to thinking of this force field as a static thing: it does not change with time. If it did, then the orbits of the planets would change with time.

Three centuries after Newton, and a hundred years ago, Einstein wondered about what new experiments imply about gravity. At that time experiments had begun to demonstrate that nothing can travel faster than light. Einstein applied this principle to gravity.

If the sun moves from one place to another, the gravity field around it must change. Since the force of gravity always pulls in towards the body, at one time it must pull in towards one position, and at another time towards another position. How would a distant planet react to the change in the sun’s position? Since nothing could move faster than light, the force of the sun’s gravity at the planet could not possibly change instantly; the planet would continue to move as if the sun had not changed its position, until this information caught up with it.

If, for some reason the sun jiggled back and forth, then the force of gravity on a distant object would jiggle back and forth, with a delay because of the speed of light. But slowly propagating jiggles are exactly what we call wave. So we figured that there are gravity waves! Such waves are exactly what were detected a year back, and announced yesterday.

So why don’t we see this all the time? First, because the motion of the planets is much slower than the speed of light. So much slower that we are not even aware of this universal speed limit. The planets behave as if the field of gravity adjusts itself instantly. Of course, particle accelerators routinely accelerate electrons and protons to very near the speed of light. But gravity is such a weak force, that we have no way of detecting the gravitational fields of these accelerated particles. This is why it requires objects 30 times as massive as the sun, moving close to the speed of light, to make gravity waves which can just barely be detected by the LIGO experiment.

Gravity waves are interesting, but it is also fascinating to listen in on a global conversation across centuries between hundreds of scientists which we marked out with simple milestones: Newton in 1687, Einstein in 1915, LIGO in 2016. The work of Newton depended on painstaking observations, calculations, and refinements of the understanding of planetary motions stretching back to the beginning of human history. From there to Einstein was a round-about path which involved Chinese and Arab observations about magnetism and electricity, leading to Faraday, Maxwell, and Bose. The trail from there to LIGO is also as fascinatingly branched and intertwined and spread across our globe, as you can judge by the names and addresses of the 1000 authors on the paper.


Author: I. J. Khanewala

I travel on work. When that gets too tiring then I relax by travelling for holidays. The holidays are pretty hectic, so I need to unwind by getting back home. But that means work.

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